In recent times, great interest has been shown in the development of vehicle headlamps which emit light within the ultraviolet wavelength range. Lamps which emit ultraviolet light have the decisive advantage of enabling fluorescent objects to be seen by the driver of the vehicle. Swedish Patent Specification No. 8704118-2, having a corresponding U.S. Pat. No. 4,970,628 to LARS A BERGKVIST, among others, describes one such headlamp. This headlamp functions to emit both visible light and ultraviolet light. The headlamp is constructed to produce a full-beam lobe with ultraviolet light and a dipped beam lobe with visible light. It is previously known to provide separate headlamps which emit essentially only ultraviolet light.
Woods glass is an example of a filter that can be used to filter-out visible light. This filter, however, has the drawback of allowing a given proportion of red light and a given proportion of blue light to pass through. Although the red and the blue light are weak, they are nevertheless found to be disturbing or discomforting to people who look into the headlamp which emits said light. Although this effect is difficult to describe, it has been judged sufficiently significant to wish to reduce or to eliminate the same. The effect is much more noticeable with headlamps which emit essentially only ultraviolet light. One of the reasons for this effect is that red and blue light is diffracted by the human eye to different extents, which can be referred to the phenomenon of bichromatic abberation. This would give rise to the aforesaid feeling of discomfort.
Attempts have been made to produce improved filters with well-defined limits. It is very difficult to produce filters with well-defined limits, and it is also doubtful that this is the correct procedure to follow, since the sensitivity of the eye does not decrease with a sharp limit between different wavelengths.
The difficulties experienced by people who look into a vehicle headlamp which emits ultraviolet light are eliminated, or at least considerably reduced, by means of the present invention.